(1) Field of the Invention
The present invention relates to a communication system and method for use in an industrial process that enables signals to be transmitted to and received from a controlled device and specifically relates to a novel electro-pneumatic instrument that receives both power and analog control signals on a single pair of conductors while also communicating digitally with the control system in a bidirectional manner on the same single pair of conductors.
(2) Description of Related Art
It is well known in industrial systems to use transducers, also called I-to-P transducers and positioners to respond to control signals for controlling the position of a valve or the like. These devices are typically powered by and receive their control signals via a single pair of conductors. These signals generally range from 4-20 milliamps DC. A maximum operating voltage is usually no more than 12 volts DC at the terminals of the device. The combined current and voltage limitations are often driven by the need to use these instruments in hazardous area where only intrinsically safe energy levels may be present.
Many devices that meet these requirements exist but most are analog in nature and do not possess the ability to transmit or receive digital information to and from other devices. For example, the Rosemount 3311 device superimposes a variable frequency on the conductor pair as a means of communicating information unidirectionally. Another example is disclosed in U.S. Pat. No. 4,633,217. The device disclosed in that patent digitally transmits information. The device disclosed in U.S. Pat. No. 4,633,217 is capable of digital transmission only. It does not receive any signals other than the 4-20 milliamp analog signal.
There are other transducer or positioner devices that communicate bidirectionally, but not via the same single pair of conductors that carry 4-20 milliamp power and the control signal. There are also many process transmitters that have the primary function of sensing process conditions rather than providing control. These devices control the 4-20 milliamp current rather than receiving it and many do communicate digitally via the same conductor pair. However, none of the control devices in the prior art utilizes a single pair of conductors to receive power and a 4-20 milliamp current control signal while also transmitting digital information to and receiving digital information from the control system.
It is important to note that transmitters control the loop current in the single pair of conductors as a normal part of their operation. Controlling the loop current independent of the DC terminal voltage of the device is equivalent to having a high DC impedance. Such a device inherently allows modulation of the loop voltage and can easily be paralleled with a like device without fundamental changes in its interface circuitry. However, for a control device to communicate with another device such as a process control system requires a novel impedance characteristic not present in transmitters. Also, paralleling of multiple control devices when communicating with a process control system requires that the impedance be able to be changed or switched to one similar to that of the transmitters.
In order for a transducer or positioner to have a sufficiently low maximum DC terminal voltage at 20 milliamps loop current and have enough power available to run a microprocessor circuit at 4 milliamps, it must have a low or negative impedance at low frequencies. In order for such a device to communicate digitally in both directions with one or more other devices, it needs to have a relatively high impedance at the communication frequencies. In order for the communication signal, which carries multiple frequency components, not to be distorted substantially, the instrument's impedance must be very high or essentially flat over the communication frequency band.
Voltage headroom is a significant technical obstacle when designing digital devices to operate under the voltage and current restrictions stated previously and still communicate digitally over the same single pair of conductors. The microprocessors have typically required 5-volt power at several milliamps. The power requirements of other circuitry can also be significant, particularly in the case of transducers and positioners where an electro-pneumatic output must be driven to perform the basic instrument function.
Although the total current required in the device usually exceeds 4 milliamps, the device itself needs to operate on 4-milliamp loop current and thus it is necessary to provide an efficient step-down power conversion in the power supply circuitry of such devices. Step-down conversion can be implemented in three basic ways. First, by linear series regulation; second, by inductor switching; or, third, by capacitor switching. Series regulation is simple and inexpensive but is very inefficient. Analog instruments are able to implement this type of regulation because of a much lower overall power requirement. Inductor switching is quite common and versatile in that it can be used to convert virtually any voltage to any other voltage. This type of conversion generates magnetic and electrical switching noise that may be undesirable and generally cannot achieve efficiencies greater than about 85 percent. Capacitor switching can be greater than 90 percent efficient and relatively quiet, but has the restriction of converting voltages in integer steps. As an example, the prior art 7660 switched capacitor voltage converter can be used only to invert, double or halve the input voltage.
The 5-volt logic of prior art could not employ switched capacitor voltage conversion because the requirement for 10-volt input to the converter could not be met and still leave enough voltage headroom for impedance control and modulation transmission without exceeding a 12 VDC terminal voltage requirement.